I live near a rock quarry that was discontinued and somehow filled with water.

I don't know exactly how this one was filled but from looking online it seems that usually what happens is that while the quarry is in use they constantly need to pump out ground water that seeps in and rain water that falls in... when they stop using the quarry and take out the pumps it eventually fills with this water.

2 Answers
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The Aruch HaShulchan (YD 201:18, quoted below) rules that a man-made well dug in the ground is a kosher mikva by virtue of its status as a spring. He notes that springs are kosher mikvas whether or not the water flows or not, and notes that in most wells the water does flow in and out of the ground, but this is not a problem. Based on my online reading I see no geological difference between a well and your case of a quarry. Thus it seems the quarry is now a kosher mikva of the highest quality.

@AvrohomYitzchok Sorry for the delay. See my answer. Note that there are different levels of water-in-the-ground as regards tahara. See Rambam Mikavaot Chapter 9 for an outline.
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Double AA♦Jul 27 '12 at 4:28